Walmart plays a heavy part in this game. They have such an outsized share of the market for Consumer Packaged Goods that they can bully manufacturers. When economics suggests the price of, say, some condiment needs to increase, Walmart simply refuses to accept the higher price. You can either lose your shelf space, or substitute a new, smaller, product format at the same price, which they will accept.

And so bottles of mustard get smaller, but stay $1.99 at Walmart. Kleenex boxes have 76 tissues instead of 88 but stay $0.99 at Walmart.